Born in Ireland, MacKenna became a ward of chancery at a young age after the death of his father.
[1] Having opted to set up practice in London instead of Dublin, MacKenna was called to the bar by the Inner Temple in January 1932 and joined the Western Circuit.
After pupillage with James Tucker, he joined the chambers of Walter Monckton, with whom MacKenna worked closely.
MacKenna was appointed to the High Court in 1961 and received the customary knighthood.
As a judge, he was critical of harsh sentencing, and in retirement was active within the Howard League.